1. Field of the Invention
The invention is generally directed to print enhancements in multi-media printers. Specifically, the invention is directed to speed and image quality advancements in thermal imaging technology, i.e., direct thermal and thermal transfer technologies in particular.
2. General Description of Background
In the medical imaging marketplace, the standard for digital image quality is wet laser imaging. In wet laser imaging, a modulated laser beam is scanned across light-sensitive film to draw the desired image on the film. The film is developed utilizing wet chemical baths, which develop and fix the image onto the film media. The image produced utilizing wet laser imaging is of high quality.
Health or medical images can also be produced or rendered utilizing a thermal printing process. In a direct thermal printing process, a thermally-responsive media is translated across a thermal printhead, and the thermal printhead is modulated to produce energy and temperature profiles to cause the media to respond to and produce desired gray levels (image densities).
Health or medical images can also be produced or rendered utilizing a thermal transfer printing process. In a thermal transfer printing process, a color-containing membrane, or “donor,” is placed or sandwiched between the thermal printhead and a “receiver” sheet of media. The donor sheet is impregnated with color or black dyes, inks, or waxes. The donor and receiver sheets are translated together across the thermal printhead, and the thermal printhead is modulated to produce energy and temperature profiles to cause the color or black carrier materials to leave the donor sheet and enter or attach to the receiver media to produce desired gray or color levels (image densities).
The thermal printhead is a linear array of thermal heating elements. Thus, an entire line of dots or pixels can be marked on the thermally-responsive media in parallel. The time it takes to mark a print line is referred to as a “line time.” Within a line time, a number of pixels, ranging from fewer than 1,000 and to more than 10,000, are marked in a row or line normal to the direction of the thermally-responsive media motion. The number of dots or pixels is typically equal to or less than the number of thermal heating elements on the printhead. The line time determines how fast the media can be moved across the printhead. In other words, the shorter the line time, the faster the media can be moved across the printhead, and the faster a full image sheet can be rendered or printed.
Thermal printing technologies have constraints on line time. In direct thermal printing, the media must be heated long enough for the heat to fully penetrate the emulsion and to allow the chemical reaction to occur. In the case of thermal transfer, the heat must be applied long enough for the dye, ink, or wax migration to occur. If the printing occurs too fast, with either technology, heat penetration and dwell times are not long enough, and grainy printing results because the heat accumulates at the peaks of the media surface and does not penetrate evenly into the low points.
Also, direct thermal printing media requires a certain dwell time in order for the intrinsic chemical reaction to go to completion. If the printing is occurring too fast, the chemical reactions cannot go to completion or they react in a manner that is complete, but result in slightly different chemical products. For example, if the printing is occurring too quickly, a purple or magenta hue will result, and if the printing is occurring too slowly, a neutral or greenish hue will result. In the medical imaging marketplace, neutral or greenish hues are favored because these hues match wet laser film. Thus, printing faster is normally at odds with a desirable color tone.
In addition, thermal transfer printing media requires a certain dwell time to allow the inks or dyes to penetrate deeply into the receiver media sheet. If the inks or dyes do not penetrate deeply enough, archival life and image stability are compromised. For instance, inks that have not penetrated deeply enough into the receiver sheet are more likely to be wiped off during handling or are more likely to diffuse out of the receiver sheet and into other sheets or materials that the receiver sheet comes in contact with.
Achieving medical image quality with a thermal printer requires compensation for the thermal storage elements in the printing system. Achieving this quality on different media types and at different print speeds requires an added measure of flexibility and performance. Thermal printing involves the application of energy to the thermal elements of the printhead. The energy applied to these thermal elements is not completely transferred into the media. Some or most of the energy applied to the thermal elements via the pulse streams are transferred from the thermal elements into the substrate of the printhead and its heatsink and then out of the heatsink through the use of forced or convected air. Because of this unintended transfer, the printhead components rise in temperature and have temperature gradients. This means that the thermal elements have different operating conditions when heating is performed for marking a line of an image. These conditions include, for example, the initial temperature of the thermal elements, the temperature of the substrate and printhead constructions behind the thermal elements, the rate of heat flow out of the thermal elements into their surroundings, and the fraction of heat that flows into the media versus the substrate. All of these factors affect marking density and must be counteracted if acceptable diagnostic medical printing is to result.
Artifacts that result from these thermal storage behaviors include the following: 1) large gradients from the top of the page to the bottom in which the measured density increases down the page as the printhead assembly components heat up; 2) vertical smearing effects in which a dark area of an image heats a local area of the printhead, causing subsequent marking for succeeding rows to be darker. This is especially noticeable when a dark area is followed by a light area or vice-versa; 3) horizontal crosstalk in which heat is transferred along the length of the printhead, causing the marking in one area of the page to affect density in neighboring areas of the page; 4) horizontal crosstalk in which the energy applied to individual thermal elements is shared with neighboring thermal elements based on temperature differences, resulting in loss of fine-resolution contrast; 5) pixel-sized artifacts in which a fast transition from light to dark or dark to light is required, and the thermal mass of the thermal elements does not allow such a fast transient (this is most visible when printing very fine detail, especially textual information in the image); and 6) artifacts resulting from the increase in printhead bow (curvature along its length) as the printhead heats up during the course of marking a page of media.
Traditional approaches to compensating for thermal memory in the thermal elements involves the application of a fixed matrix of coefficients that are multiplied by the image energy values that border and precede the pixel in question. This is commonly known in the art as “dot history compensation.” The products of these energy values and coefficients are summed and the result is applied to the pixel energy value in question to provide a compensation that takes neighboring energy values and historical energy values into account. While this approach can provide limited success, it does not provide density uniformity that is in line with medical market expectations. It is also difficult to tune and make effective for all types of images produced. This is because the size of the matrix is limited for performance reasons, limiting the size of the artifacts that can be compensated, because determining effective values for the matrix coefficients is difficult and increases exponentially with the number of historical pixels to be considered, and because it does not effectively account for the physical properties of the thermal printing system.
Medical diagnostic imaging requires print engines to be able to produce large numbers of distinct gray levels ranging from minimum to maximum density. The gray levels are produced by applying a large number of on and off pulses to each printhead thermal element within each line time. The large number of pulses may be referred to as a pulse stream. For example, in order to produce 512 gray levels per image pixel, a multi-media printer must be able to provide at least 512 pulses to the thermal elements over and above the pulse times used to reach the minimum marking energy or blush energy. The time duration of a single pulse is referred to as a pulse time. The minimum marking energy or blush energy may be referred to as Dmin. Normally, this results in 1024 pulse times within a line time. In order to achieve 4096 gray levels, which is the standard that wet imager manufacturers claim is produced by the wet laser film process, an equivalent of 8000 pulses would have to be produced in a line time.
An alternative for achieving the 4096 gray levels with less than 8000 pulses involves utilizing a bias pulse with a length equivalent of 4096 pulses and applying the bias pulse to all thermal elements, regardless of gray level, to get the thermal elements up to blush temperature. Thus, streams of 4096 pulses could be applied to each data line for each thermal element of the printhead in parallel. This would require data transmission of 4096 bits of information per printhead thermal element within a line time. Unfortunately, the current state of printhead technology does not allow 4096 bits of information to be transmitted within a 5 ms line time because this large amount of data would have to be transferred to the printhead in too small of a time. This alternative would require 500 data lines running at 16 Megahertz (16 Mhz), in parallel. The use of this many data lines is expensive. In addition, data integrity and emission issues are also present due to the number of data lines operating at such a high speed.
Therefore, it is desirable to develop technologies that provide faster than normal printing and also still provide clinically acceptable tone on different media types. The technology also should produce minimal graininess on many media types.
These technologies would allow use of higher-resolution printheads. An embodiment of the new technologies allows a 14×17 inch film to be marked within 25 seconds at 500 dots per inch using two 16 Megahertz data lines while still providing 4096 gray levels. This is compared to current technology which would require 1200 data lines running at 16 MHz to be connected to the printhead. At these speeds, and allowing extra time to pick, feed, and eject the media, this embodiment provides the printing of 90 or more films per hour at resolutions higher than 500 dpi. In addition to the very high print speed this constitutes a reduction in data line count of more than two orders of magnitude.